DESCRIPTION (Investigator's Abstract): This is a revised application from an established investigator to characterize the anatomical distribution of two isoforms of glutamate decarboxylase, an essential enzyme for GABA synthesis and to analyze and compare the development of three (-subunits of the GABA-A receptor. The hypothesis is that one of the GAD isoforms -GAD67- plays a "predominant role in the earliest phase of development" The PI will use anatomical methods address four specific aims: (1) to determine if early-appearing hippocampal GAD-containing neurons are analogous to cortical marginal/subplate neurons. The PI will determine the developmental expression of GAD mRNAs; the birthdates of GAD containing neurons; the possibility of cell death among GAD neurons and if early arriving afferents are in close proximity to GAD neurons. (2) The postnatal development of GAD65 and 67 neurons will be described (3) Describe the time course of development and relation to GAD- containing neurons of three (-subunits of the GABA-A receptor. (4) Characterize the ultrastructural localization of the two forms of GAD at two "key" phases of hippocampal development.